Superplastic forming is a process which utilizes the properties of certain materials that can be extensively strained at relatively low stress levels when heated to an elevated temperature known as the superplastic forming temperature. Certain formulations of aluminum, rolled in a certain pattern, exhibit superplacticity at superplastic temperatures, as do titanium and some titanium alloys, certain stainless steels and some super alloy materials. All of these materials have been used to form low tolerance parts with little or no residual stress, which would have been difficult or impossible to achieve with prior art metal forming processes.
The forming of tubular structures by superplastic forming in the past has been performed by superplastic forming two longnitudial halves of the part as separate pieces and welding the two pieces together to make the final part. This process can produce a satisfactory part, but it is costly and great care must be taken to avoid quality problems, especially if the part must be capable of withstanding gas pressure.
An ideal method of forming tubular parts by superplastic forming would be to begin with a tubular blank and to superplastically form the tubular blank against inside cavity surfaces in a die having an internal configuration like the external shape of the final part. This process would eliminate the cost of making the parts in two halves then welding the halves together and would result in a seamless part having excellent part quality and minimal variation from part to part.
A conventional superplastic forming process utilizes a sheet of superplastic material which is captured around its peripheral edge between a die base and a die lid. The sheet is heated to superplastic forming temperature in the die and the sheet is then strained into contact with the surface of the die cavity by gas pressure introduced under the die lid. The tubular analog to the flat sheet superplastic forming process, that is, using the forming gas pressure to form a tube of superplastic material against internal surfaces in a die cavity, would require that the tube be sealed around the peripheral edges of both ends of the tube to establish a pressure zone inside the tube for straining the tube material outward into contact with the inside surfaces of the die cavity. The sealing of the tube in a superplastic forming die can be complicated and unreliable because of the various factors involved in superplastic forming, including the very high temperatures at which certain materials become superplastic and the high pressure of the forming gas required to strain the material, even at a superplastic temperature. Thus, there has long been an unfulfilled need in the art to provide a simple, inexpensive and reliable method and an apparatus for sealing the ends of a superplastic tube in a superplastic forming die for superplastic forming of the tube.